1. Field of the Invention
This invention relates to an engine control system for a hybrid vehicle, and more particularly, to an engine control system which can prevent vibration of a vehicle body which occurs until the engine speed becomes xe2x80x9c0xe2x80x9d after deceleration fuel supply cut is performed.
This application is based on Japanese Patent Application No. 11-62412, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, hybrid vehicles which carry motors as power sources for driving the vehicles in addition to engines are known. Hybrid vehicles are divided into series hybrid vehicles and parallel hybrid vehicles. In the series hybrid vehicles, the engine drives a generator whose electric power output is used to drive the motor, which in turn drives the wheels. In the parallel hybrid vehicles, the motor coupled to the engine assists the rotation of the drive shaft of the engine while charging a battery with electric energy using the motor itself as a generator or using a separate generator.
Because the engines of such hybrid vehicles can constantly run within the engine-speed range of a high fuel mileage and a low emissions level or their drive loads can be reduced, the hybrid vehicles can achieve lower fuel consumption and lower emissions levels than conventional engine vehicles.
As disclosed in, for example, Japanese Unexamined Patent Application, First Publication Hei 8-317505, some of those hybrid vehicles are designed to be capable of stopping the operation of the engine in accordance with predetermined driving conditions.
Because of the ability to stop the operation of the engine, such a hybrid vehicle is superb in preventing over-charging of the battery or achieving a further improvement in fuel consumption but has the following problem.
If the fuel supply is cut before the engine is stopped, the engine speed gradually decreases after the fuel is cut. Before the engine speed finally becomes xe2x80x9c0xe2x80x9d, the frequency resonance points of both the vehicle body and the drive system may coincide with each other depending on the vehicle speed and the number of rotations of the drive system. This inevitably causes vibration of the vehicle body, which is likely to make the driver feel uncomfortable.
Accordingly, it is an object of the present invention to provide an engine control system for a hybrid vehicle, which can eliminate uncomfortable vibrations of the vehicle body by allowing the engine to swiftly leave the state where the engine speed causes such resonance.
To achieve this object, according to one aspect of this invention, an engine control system for a hybrid vehicle, having an internal combustion engine (e.g., an engine E in one embodiment) and an electric motor (e.g., a motor M in the embodiment) as driving force sources, for cutting fuel supply to the engine when the vehicle decelerates and permits stopping and restarting of the engine in accordance with predetermined drive conditions, comprises: a fuel cutter (e.g., steps S129 and S320 in the embodiment); and an engine speed detector (e.g., an engine speed sensor S5 in the embodiment) for detecting an engine speed (e.g., an engine speed NE in the embodiment), wherein stopping the engine is initiated by cutting the fuel supply by the fuel cutter, and when the engine speed becomes equal to or lower than a predetermined speed (e.g., an engine-stopping/enforced-regeneration-starting upper speed limit NEIDLSTH in the embodiment), the engine is stopped by operating the motor as a generator and by cutting the fuel supply.
When the fuel supply to the engine is stopped by the fuel cutter and the engine speed falls to or below a predetermined speed, the vehicle body tends to vibrate. At this point of time, the motor is permitted to work as a generator to apply a load to the engine and the fuel is cut, thereby promptly making the engine speed xe2x80x9c0xe2x80x9d.
The predetermined drive conditions for stopping the engine are, for example, such that on the premise that the engine can be started by the motor (e.g., when a motor-start enable/disable flag F_MOTSTB is xe2x80x9c1xe2x80x9d in step S106 in the embodiment), the amount of the remaining battery charge (or the state of charge) SOC is equal to or above an over-discharge area (e.g., an energy-storage-zone flag F_ESZONE is xe2x80x9c0xe2x80x9d in step S107 in the embodiment), and the engine coolant temperature TW is equal to or higher than a predetermined level (e.g., the coolant temperature TW is equal to or higher than a coolant-temperature lower limit TWFCMG in step S108 in the embodiment).
In this engine control system, the predetermined speed may be set in a resonance frequency band of a vehicle body and a drive system (e.g., between the engine-stopping/enforced-regeneration-starting upper speed limit NEIDLSTH and an engine-stopping/enforced-regeneration-starting lower speed limit NEIDLSTL), which is equal to or lower than an idling speed.
With this structure, when vibration with a large amplitude tends to occur at a speed equal to or lower than the idling speed, the motor is permitted to work as a generator to apply a load to the engine, thereby promptly making the engine speed xe2x80x9c0xe2x80x9d. This can make extremely short the time during which the engine speed falls within the resonance frequency band, and can allow the engine speed to quickly leave from the resonance frequency band.
In the above engine control system, a fuel supply restarting engine speed for restarting fuel supply is set in the fuel cutter, and once engine stop control is initiated, the engine stop control is maintained and the fuel cutting continues even when the engine speed falls to the fuel supply restarting engine speed. This can suppress the occurrence of uncomfortable vibrations to an extremely short time.